A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive metal compound (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Defects, such as small particles or other geometrical aberration, for example, due to surface defects on an article, for example, an article with a patterned surface, such as EUV reticles, will randomly scatter the light. By imaging a fraction of scattered radiation from the article on a detection device, the defects will light up as bright spots. The intensity of the spots is a measure of the size of the particle. Also, a substrate surface of a substrate to be patterned by a radiation beam in an EUV apparatus should be as particle free as possible. In this regard, a need arises for inspecting an article for defects. This may be done while filtering out higher order diffractions that arise from periodic patterns that can be present on the article. This filtering can be done by introducing a spatial filter in a Fourier plane of an imaging system that images the aforesaid fraction. However, this introduces a need for coherent light, so that the diffractions can be filtered. Generally, the use of coherent light may introduce a problem of possible speckle formation, which may arise due to surface irregularities, even when in the order of less than 1 nm.